Human Mini-Brains Growing Inside Rat Bodies Are Starting to Integrate

"We are entering totally new ground here."

Stem cell technology has advanced so much that scientists can grow miniature versions of human brains — called organoids, or mini-brains if you want to be cute about it — in the lab, but medical ethicists are concerned about recent developments in this field involving the growth of these tiny brains in other animals. Those concerns are bound to become more serious after the annual meeting of the Society for Neuroscience starting November 11 in Washington, D.C., where two teams of scientists plan to present previously unpublished research on the unexpected interaction between human mini-brains and their rat and mouse hosts.

In the new papers, according to STAT, scientists will report that the organoids survived for extended periods of time — two months in one case — and even connected to lab animals’ circulatory and nervous systems, transferring blood and nerve signals between the host animal and the implanted human cells. This is an unprecedented advancement for mini-brain research.

“We are entering totally new ground here,” Christof Koch, president of the Allen Institute for Brain Science in Seattle, told STAT. “The science is advancing so rapidly, the ethics can’t keep up.”

DMT receptors in mini-brain organoids.

That mini-brains can even be grown in the lab is a huge advancement in the first place, as they have many of the same characteristics as living human brains that are in the early stages of development. Though they’re not “alive” in the same sense that you and I are, they grow and are organized into different layers like our brains are. They even react in similar ways to stimuli like psychedelic drugs. Organoids are poised to revolutionize research on the human brain since scientists can perform tests on them that would be unethical to attempt on living humans.

Scientists have debated whether these brains are “conscious,” but the fact that they could be successfully implanted in lab animals raises a whole new set of ethical concerns for the researchers who work with them. One of the major concerns in the mini-brain scenario is that these organoids could grow to more advanced levels within lab animals, making the debate about mini-brain consciousness much more urgent.

In Salk Institute experiments, human stem cells were pipetted into a pig embryo.

Putting human brain structures into non-human animals creates a thorny ethical area that raises people’s fears about medical research going too far into unfamiliar territory — and too quickly. It’s likely to be a recurring theme in this field, too. In January, Salk Institute researchers developed human-pig chimeras, creating the possibility that pigs with human brain cells might also develop human consciousness.

STAT also reports that a third lab, in addition to the two presenting at the Society for Neuroscience meeting, has successfully connected human brain organoids to blood vessels. This attempt veered into such challenging ethical territory, though, that the lab reportedly paused its efforts.

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